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Induction heating is apparently an effective solution for heating the mold surface without increasing the molding cycle period. However, the main unresolved problem of using induction heating in injection molding is the non -uniform temperature distribution on the heating surface. The various causes of differences in heating temperature include the coil turn space, heating distance, cavity geometry, magnetic interference, and the proximity effect. For instance, different directions of electric current a long induction coils can cause an on-uniform temperature distribution by introducing a repulsive proximity effect. For a practical coil design, however, opposite directions of coil current are difficult to avoid.
In a study of the effect of repulsive proximity on non-uniform temperature distribution, Sung et al. [20] compared temperature uniformity in the heating surfaces of different induction heating coils . The experimental results showed poor heating efficiency in the single-layer coil with opposite current directions. However, the double-layer reciprocating coil and the coil with magnetic flux concentrator efficiently increased heating speed and provided uniform temperature distributions. Huang [2 1] designed a multilayer coil for improving heating efficiency and temperature uniformity in a conventional single-layer coil. The experimental results showed that the multilayer coil has a more uniform temperature distribution and better heating efficiency compared to the conventional single-layer coil.
Although, the double-layer reciprocating coil can avoid the heating layer (i.e., the layer near the heating surface) with opposite current di-re c ti on , the double -layer coil must have reduplicate coil length of single- layer reciprocating coil . However , the increased coil length increases coil manufacturing cost and energy consumption. The local multilayer coil has a more uniform temperature distribution and better heating efficiency; applications of the local multilayer coil are limited to spiral -like forms . Applying magnetic flux concentrator to control magnetic flux field and eliminate proximity effect is the most popular method in the induction heating process, but fails to completely isolate the conflict in g magnetic fluxes when opposite current coils were induced
Induction heating of metal is performed by electromagnetic induction. According to the Faraday law and the Lenz law, passing an electrical alternating current through heating coils produces an alternating magnetic field that can be used to heat an object. If processed magnetic or non-magnetic conductive workpieces are placed in the alternating magnetic field established by the heating coils, cutting-of- flux produces current at different depths. The workpieces' resistance and the flow of eddy currents therein generate heating power [22] . Induction heating exploits two effects, the Joule effect, which is based on hysteresis loss and eddy current loss, and the electromagnetic effect, which mainly comprises the skin effect and the proximity effect. These effects are described further below.
Passing an alternating current through a conductive wire generates a Magnetic field both in side and outside the conductor. Varying the current then causes a variation in the magnetic field in the conductor,which generates an induction electromotive force and, thus, a current in the conductor. The current in the conducting wire section is distributed as follows: the current near the center of the conducting wire is very small whereas that at the surface is relatively large. The cause of the difference is known as the skin effect. In induction heating, a high -frequency current passing thro ugh the coil produces the largest induction eddy current at the workpiece surface. The eddy current declines exponentially with distance from the surface.
The proximity effect can be observed when conductive wires are placed in the vicinity of each other . The magnetic fields of the two conductive wires affect each other, and the associated magnetic field on the mold surface changes [ 23] . Fig. 1 shows the effect of current direction on magnetic flux line. When two coils have an identical current direction , the magnetic flux lines are combined through out the mold surface, which enables uniform heating of the mold surface.